Analysis on electromechanical coupling coefficients in AlN-based bulk acoustic wave resonators based on first-principle calculations

We discuss the effect of aluminum nitride (AlN) film stress on the electromechanical coupling coefficient (k²) of film bulk acoustic resonators (FBAR) based on first-principle calculations and experiments. Our calculations indicate that the compressive stress of AlN film induces the expansion of the c-axis lattice constant and affected the piezoelectric constant of AlN. We found that the piezoelectric constant of AlN and k² of FBAR estimated using first-principle calculation increase with decreasing compressive stress of AlN films. We also confirmed from experiments that the piezoelectric constant increased from 6.5 to 7.1pC/N due to the decrease in compressive stress. Finally, we fabricated air-gap type FBAR that have different AlN film stresses. As a result, k² improved from 6.0 to 6.5% by controlling the film stress from -1200 to 0MPa.